The present invention relates to micro electro-mechanical systems (MEMS) devices and more specifically to enhancing the reliability and lifetime of these devices when operating in the presence of UV illumination.
Micro electro-mechanical system (MEMS) devices are usually small structures, which rely on mechanical motion to function. The digital micro-mirror device (DMD) is a MEMS structure comprised of an array of tiny micro-mirrors, which are attached to posts extended up from the substrate by means of compliant torsion hinges. Electrostatic fields between the mirrors and memory cells located in the underlying semiconductor substrate cause the mirrors to rotate (tilt) in either a positive or negative direction. These devices have found wide application as spatial light modulators (SLM) and optical switching devices.
Since there is motion involved, special lubricants, passivants, getters, and/or other gases are often included in the package headspace (space inside package around device) around the device to assure smooth (proper) operation of the device. For example, without special lubricants the micro-mirrors of a DMD may stick causing a lubricants the micro-mirrors of a DMD may stick causing a defect in the device. This sticking problem can result from hydration, surface tension, tribology, or any number of other effects for which these special lubricants and getters help prevent.
There are a number of applications requiring UV illumination, such as photolithography and photo-finishing, where MEMS devices are highly effective, due to their optical performance. However, the headspace chemistry inside the MEMS package often contains sufficient amounts of fluorocarbon, chlorocarbon, fluorochlorocarbon, and hydrofluorochlorocarbon modalities that when exposed to UV flux can become reactive, due to photochemical activation, damaging the surfaces of the MEMS device and the package optical window.
What is needed is a way to eliminate this surface degradation in MEMS devices, caused by photochemical activation, when used in UV illuminated applications. This invention describes a new use process for enhancing the reliability, and hence the lifetime of MEMS devices in UV exposed environments by properly coating the device and window surfaces with a metal halide.
This invention discloses a process for precluding damage to a MEMS device, used in a UV illuminated application, due to a photochemical activation between the optical UV flux and package gas constituents. These gases come from out-gassing of various lubricants and passivants put in the device package to prevent sticking of the MEMS device""s moving parts. When these gases are exposed to UV flux, various chemical radicals can be generated that can etch or otherwise damage the surfaces of the device and the package window.
The essence of this invention is to coat the exposed surfaces of the MEMS device and window surface with various metal-halides to eliminate this photochemical activation and therefore significantly extend the reliability and lifetime of the MEMS device. These coating prevent the Si and SiO2 of the MEMS device, and the SiBO7 of the package""s optical window from being exposed to UV flux, thereby eliminating the formation of such volatile compounds as SiF4, SiCl4, BF3, and BCl3, any of which has the tendency to destroy the MEMS device.